1. Field of the Invention
This invention relates to material removal from a member surface, and more particularly to a method and apparatus for making, inspecting and controlling the position of small holes in a surface of a member, such as a sheet or panel member.
2. Description of the Prior Art
More sophisticated sound attenuation members, for example sound suppression panels for use in gas turbine engines include one or more components in which specifically sized, shaped and spaced openings are disposed. Such openings can be perforations, slots, etc. U.S. Pat. No. 3,542,152-Adamson et al., issued Nov. 24, 1970; U.S. Pat. No. 3,819,008-Evans et al., issued June 25, 1974; and U.S. Pat. No. 4,130,175-Hehmann, issued Dec. 19, 1978, all assigned to the assignee of this invention, show examples of such members. Frequently, such arrangements include one or more sheet members for example at one face or within the structure or both, with a plurality of holes, slots, etc. of a particular size and arrangement selected to cooperate with other portions of the panel to attenuate predetermined sound waves. One such perforated, reinforced plastic sheet member is described in U.S. Pat. No. 3,704,194-Harrier, patented Nov. 28, 1972; another is shown in U.S. Pat. No. 3,787,546-Pratt et al., issued Jan. 22, 1974, all assigned to the assignee of this invention. As shown by these references, sheets of reinforced plastic have been perforated by pressing the sheet over a mandrel. Other means for perforating such members includes mechanical drilling and the use of high energy sources such as electron beam and laser.
The manufacture of such perforated sheets including very small, closely spaced holes of a uniform preselected pattern has been difficult and costly: electron beam drilling when used with fiberglass reinforced plastic laminated sheet has been seen to oxidize the plastic binder, such as epoxy resin, to the extent that the material is no longer functional as a laminate. A laser has been used to drill holes in various materials. However, because of slight variations in the consistency and thickness inherent in reinforced plastic materials, hole patterns have not been practical to inspect and control as the holes are being produced. Use of an intermittent scanning beam in cooperation with a separate electron beam for drilling has been described in U.S. Pat. No. 3,192,318-Schleich et al., patented June 29, 1965. Such a method and apparatus employs a means to deflect the scanning beam over a master drawing with the video pulses supplied by the scanning beam controlling the intensity of the electron beam impinging on the workpiece. However, such a method employing intermittent inspection after a series of holes have been generated does not provide the opportunity to correct improper positioning of holes as they are being generated. In addition, a separate scanning beam is provided to light the work area.